The invention relates to a fabric for the sheet forming section of a papermaking machine, e.g. a so-called sheet former. The fabric consists of a double-layer fabric, i.e. two layers of transverse threads are provided, and the longitudinal threads interweave with each of the two layers of transverse threads. Both on the running side and on the paper side, the transverse threads are predominantly visible. In each repeat, the longitudinal threads interweave twice with the upper layer of the transverse threads. On the paper side the transverse threads overly at least 80% of the longitudinal threads. The fact that the number of transverse threads on the running side is only half that on the paper side is compensated by the greater diameter of the transverse threads of the lower layer.
In the sheet forming fabric disclosed in German Patent Publication No. 2,706,235 of 9-8-1977 (FIG. 2F), the longitudinal threads have a relatively flat course, despite two points of interweaving with the upper layer in the interior of the fabric, with the consequence that the woven seam by which two fabric ends of flat woven fabrics are joined to form an endless fabric, has a relatively low strength. Moreover, cleaning with high pressure water jets from the running side soon destroys the woven seam. The locations where the longitudinal and warp threads meet within the woven seam are distributed at random within the woven seam. The warp thread ends lie exposed side by side at the meeting points, i.e. each meeting point is an interruption of the warp thread, so that the transverse or weft thread of the lower layer is not held at the meeting point of the warp thread ends and is more intensely worn over a width corresponding to two weave repeats. The unwoven free warp thread ends are shredded and destroyed very soon when the papermachine fabric is cleaned with high pressure water jets.
In European Patent Publication No. 30 490 of 6-17-81, a double-layer sheet forming fabric is disclosed in which the longitudinal threads interweave twice per each repeat with the upper layer of transverse threads, but only once with the lower layer of transverse threads. This sheet forming fabric has the same number of transverse threads in the upper and in the lower layer so that the two points if interweaving with the upper layer of transverse threads shorten the transverse thread floats on the paper side. As a consequence, it is difficult to remove the paper web from the sheet forming fabric.
A similar sheet forming fabric is disclosed in German Patent Publication No. 2,263,476 of 3-7-74, where the longitudinal threads are interwoven two or three times into the upper layer. In the lower layer, the longitudinal threads extend under two or three transverse threads, so that they are subject to high wear. According to German Patent Publication No. 2,540,490 of 4-22-76, this is remedied in that each longitudinal thread interweaves at the most with every sixth transverse thread in the lower layer. However, there still remains the disadvantage of low dehydration capacity and difficulties in removing the sheet, especially in the manufacture of tissue paper.
In the manufacture of paper on twin wire paper machines, it is a nuisance that the interior of the papermachine fabric fills with water flowing through, and at the point of deflection of the fabric, said water is forced out of the interstices of the lower layer. Due to the high speeds of 1500 to 1800 m/min at which the twin wire formers are operated, the issuing water forms a dense mist at the first fabric deflection point.
In U.S. Pat. No. 4,564,042, a double layer sheet former for a papermaking machine is disclosed wherein the longitudinal threads are interwoven with the lower layer twice in each repeat, the first time together with the preceding longitudinal thread, and the next time together with the next following longitudinal thread, the longitudinal thread extending between two points of interweaving over at least one transverse thread of the lower layer. This mode of interweaving is to ensure that the sheet forming fabric is a transverse thread runner and accordingly has a longer service life.